Several types of electronic components are implemented with a circuit, which is integrated in a chip of semiconductor material. The chip is typically mounted on a carrier, so as to protect the chip from mechanical stresses, and is then encapsulated in a package. The chip carrier includes an insulating substrate with conductive tracks; each track is bonded to a corresponding terminal of the chip, and ends with a contact pad, typically for connection to a printed circuit board. Likewise, printed circuit board generally comprises several conductive layers formed in an insulating material, adapted to transmit signals between several electronic devices or between electronic devices and connectors.
When switching speeds of devices goes above 1 GHz clock rate, there is a need to no longer consider electrical signal transmission as a simple point to point transmission on a track but rather as the propagation of an electromagnetic wave supported by a current on a circuit trace. Such traces on electronic device carriers (chip carriers and printed circuit boards), also called transmission lines, represent a system comprising at least two conductive paths with specific properties (relation between transmission line width, distances between transmission lines, dielectric thickness between transmission lines and reference planes). These transmission lines comprise a conductive signal track or trace and another track and/or conductive plane, formed in close proximity and connected to a reference voltage or ground, for shielding the signal track from electromagnetic interference. The wave propagates along a transmission line defined by the signal track and an underlying reference voltage or ground plane, forming a complete loop path for the signal current. When the chip works at a high frequency, e.g. more than 1 GHz, the influence of the electronic device carrier may severely affects the performance of the electronic system as a whole.
Particularly, any discontinuity (or transition) in the transmission line, such as any change in structure, material properties and design features, generates a reflected wave. Moreover, the system includes stray structures (capacitors, inductors and resistors), which act as low pass filters for the transmitted signal. As a consequence, the integrity of the electromagnetic wave propagated along the transmission line is not preserved.
The transmitted signal, switching between a low voltage (logic value 0) and a high voltage (logic value 1), generates a square-shaped wave. Due to all discontinuities in the transmission line, this wave undergoes degradation and is generally received as a pseudo-sinusoidal wave. The quality of the transmitted wave can be visualized by a so-called “eye diagram”, which plots the value of the received signal as a function of the phase of a clock signal controlling the electronic device. The above described discontinuities in the transmission line reduce the opening of the eye diagram; therefore, it is quite difficult to understand if a switching transition has actually taken place or if the shift of a signal baseline is due to a background noise.
These drawbacks are particular acute in modern electronic systems working with a reduced level of a power supply voltage (down to 1.2 V). In this case, there is a very low margin to discriminate between the logic value 0 (0V) and the logic value 1 (1.2V).
Moreover, the continuous trend towards miniaturization of electronic devices requires a reduction in the dimensions of chip carrier and printed circuit board conductive tracks. However, the impedance of the transmission line must be maintained at a desired value which optimizes the performance of the electronic device (typically 50Ω). Therefore, it is necessary to use a very thin dielectric layer between the conductive tracks and the ground plane (since the impedance is inversely proportional to the track width and directly proportional to the dielectric layer thickness). The short distance between the conductive tracks and the ground plane increases the value of a corresponding stray capacitance; as a consequence, the bandwidth of the transmission line is strongly reduced.
Therefore, as the quality of the transmission in the electronic device carrier, i.e. chip carrier or printed circuit board, is degraded it can cause the electronic device to operate at a frequency far lower than the working frequency which is afforded by the chip.